R'Man has nailed it I think. In the days of Card & Felt/Fiber wads with a short shell the wad column often did not bridge the gap between shell mouth & cone allowing gas to blow by the wads, which could on occasion give a reduction of pattern. Another phenomenon ones see in old writings was "Balled Shot" in which this gas blow-by fused a number of shot together. All of this has been virtually eliminated by the plastic obturating wad cup.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra